William Carter From The Bureau: Xcom Declassified

To get the hidden hero William Carter, just go to your Barracks, select a hero unit, go to the Customize option, and change his name to William Carter (with William in the First name area and Carter in the Last name area). He is an Assault Class pre-equipped with Titan Armor, Alloy Cannon, and Plasma Pistol.

Xcom Heroes

When going to your soldiers Customize place, enter any of the above first names and last names in the respective place(they must be spelled and capitalized correctly, but nicknames are automatically put in. Note that putting any of these heroes in the game disables achievements permanently(the game warns you), and disables customization for them. However, each of them come with the best of their class, but there equipment cant be used by other squad members or heroes until properly researched.